Stéphanie Cazaux

The hot core around the low-mass protostar IRAS 16293-2422 : Scoundrels rule!

While warm dense gas is prevalent around low-mass protostars, the presence of complex saturated molecules—the chemical inventory characteristic of hot cores—has remained elusive in such environments. Here we report the results of an IRAM 30 m study of the molecular composition associated with the low-mass protostar IRAS 16293-2422. Our observations highlight an extremely rich organic inventory in this source with abundant amounts of complex O- and N-bearing molecules such as formic acid, HCOOH, acetaldehyde, CH3CHO, methyl formate, CH3OCHO, dimethyl ether, CH3OCH3, acetic acid, CH3COOH, methyl cyanide, CH3CN, ethyl cyanide, C2H5CN, and propyne, CH3CCH. We compare the composition of the hot core around this low-mass young stellar object with those around massive protostars and address the chemical processes involved in molecular complexity in regions of star formation.

Formation on grain surfaces

The most abundant interstellar molecule, H-2, is generally thought to form by recombination of H atoms on grain surfaces. On surfaces, hydrogen atoms can be physisorbed and chemisorbed and their mobility can be governed by quantum mechanical tunneling or thermal hopping. We have developed a model for molecular hydrogen formation on surfaces. This model solves the time-dependent kinetic rate equation for atomic and molecular hydrogen and their isotopes, taking the presence of physisorbed and chemisorbed sites, as well as quantum mechanical diffusion and thermal hopping, into account. The results show that the time evolution of this system is mainly governed by the binding energies and barriers against migration of the adsorbed species. We have compared the results of our model with experiments on the formation of HD on silicate and carbonaceous surfaces under irradiation by atomic H and D beams at low and at high temperatures. This comparison shows that including both isotopes, both physisorbed and chemisorbed wells, and both quantum mechanical tunneling and thermal hopping is essential for a correct interpretation of the experiments. This comparison allows us to derive the characteristics of these surfaces. For the two surfaces we consider, we determine the binding energy of H atoms and H-2 molecules, as well as the barrier against diffusion for the H atoms to move from one site to another. We conclude that molecular hydrogen formation is efficient until quite high (similar to 500 K) temperatures. At low temperatures, recombination between mobile physisorbed atoms and trapped chemisorbed atoms dominates. At higher temperatures, chemisorbed atoms become mobile, and this then drives molecular hydrogen formation. We have extended our model to astrophysically relevant conditions. The results show that molecular hydrogen formation proceeds with near unity efficiency at low temperatures ( T less than or equal to 20 K). While the efficiency drops, molecular hydrogen formation in the ISM can be very efficient even at high temperatures, depending on the physical characteristics of the surface.

Complex Molecules in the Hot Core of the Low-Mass Protostar NGC 1333 IRAS 4A

We report the detection of complex molecules (HCOOCH3, HCOOH, and CH3CN), signposts of a hot core-like region, toward the low-mass Class 0 source NGC 1333 IRAS 4A. This is the second low-mass protostar in which such complex molecules have been searched for and reported, the other source being IRAS 16293-2422. It is therefore likely that compact (a few tens of AU) regions of dense and warm gas, where the chemistry is dominated by the evaporation of grain mantles and where complex molecules are found, are common in low-mass Class 0 sources. Given that the chemical formation timescale is much shorter than the gas hot-core crossing time, it is not clear whether the reported complex molecules are formed on the grain surfaces (first-generation molecules) or in the warm gas by reactions involving the evaporated mantle constituents (second-generation molecules). We do not find evidence for large differences in the molecular abundances, normalized to the formaldehyde abundance, between the two solar-type protostars, suggesting perhaps a common origin.

Molecular Hydrogen Formation in the Interstellar Medium

We have developed a model for molecular hydrogen formation under astrophysically relevant conditions. This model takes fully into account the presence of both physisorbed and chemisorbed sites on the surface, allows quantum mechanical diffusion as well as thermal hopping for absorbed H atoms, and has been benchmarked versus recent laboratory experiments on H2 formation on silicate surfaces. The results show that H2 formation on grain surfaces is efficient in the interstellar medium up to some 300 K. At low temperatures (≤100 K), H2 formation is governed by the reaction of a physisorbed H with a chemisorbed H. At higher temperatures, H2 formation proceeds through a reaction between two chemisorbed H atoms. We present simple analytical expressions for H2 formation that can be adopted to a wide variety of surfaces once their surface characteristics have been determined experimentally.

Near-Arcsecond Resolution Observations of the Hot Corino of the Solar-Type Protostar IRAS 16293–2422*

Complex organic molecules have previously been discovered in solar-type protostars, raising the questions of where and how they form in the envelope. Possible formation mechanisms include grain mantle evaporation, the interaction of the outflow with its surroundings, and/or the impact of UV/X-rays inside the cavities. In this Letter we present the first interferometric observations of two complex molecules, CH3CN and HCOOCH3, toward the solar-type protostar IRAS 16293-2422. The images show that the emission originates from two compact regions centered on the two components of the binary system. We discuss how these results favor the grain mantle evaporation scenario, and we investigate the implications of these observations for the chemical composition and physical and dynamical state of the two components.

Water formation on bare grains: When the chemistry on dust impacts interstellar gas.

Context. Water and O(2) are important gas phase ingredients for cooling dense gas when forming stars. On dust grains, H(2)O is an important constituent of the icy mantle in which a complex chemistry is taking place, as revealed by hot core observations. The formation of water can occur on dust grain surfaces, and can impact gas phase composition. Aims. The formation of molecules such as OH, H(2)O, HO(2) and H(2)O(2), as well as their deuterated forms and O(2) and O(3) is studied to assess how the chemistry varies in different astrophysical environments, and how the gas phase is affected by grain surface chemistry. Methods. We use Monte Carlo simulations to follow the formation of molecules on bare grains as well as the fraction of molecules released into the gas phase. We consider a surface reaction network, based on gas phase reactions, as well as UV photo-dissociation of the chemical species. Results. We show that grain surface chemistry has a strong impact on gas phase chemistry, and that this chemistry is very different for different dust grain temperatures. Low temperatures favor hydrogenation, while higher temperatures favor oxygenation. Also, UV photons dissociate the molecules on the surface, which can subsequently reform. The formation-destruction cycle increases the amount of species released into the gas phase. We also determine the timescales to form ices in diffuse and dense clouds, and show that ices are formed only in shielded environments, as supported by observations.

Molecular hydrogen formation on dust grains in the high-redshift universe

We study the formation of molecular hydrogen on dust grain surfaces and apply our results to the high-redshift universe. We find that a range of physical parameters—in particular dust temperature and gas temperature, but not so much dust surface composition—influences the formation rate of H2. The H2 formation rate is found to be suppressed above gas kinetic temperatures of a few hundred K and for dust temperatures above 500 K and below 10 K. We highlight the differences between our treatment of the H2 formation process and other descriptions in the literature. We also study the relative importance of H2 formation on dust grains with respect to molecular hydrogen formation in the gas phase, through the H- route. The ratio of the formation rates of these two routes depends to a large part on the dust abundance, on the electron abundance, and on the relative strength of the far-ultraviolet (extra-)galactic radiation field. We find that for a cosmological evolution of the star formation rate and dust density consistent with the Madau plot, a positive feedback effect on the abundance of H2 due to the presence of dust grains can occur at redshifts z ≥ 3. This effect occurs for a dust-to-gas mass ratio as small as 10-3 of the galactic value.We study the formation of molecular hydrogen on dust grain surfaces and apply our results to the high redshift universe. We find that a range of physical parameters, in particular dust temperature and gas temperature, but not so much dust surface composition, influence the formation rate of H

HD and H-2 formation in low-metallicity dusty gas clouds at high redshift

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HDO abundance in the envelope of the solar-type protostar IRAS 16293-2422

. The H

How micron-sized dust particles determine the chemistry of our Universe

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